T1: A Survival Guide

RJ-48X

In the 1970s, AT&T developed a system of codes for the different types of jacks used at customer premises. These jack codes were called Universal Service Order Codes (USOCs), and jack specifications were known as Registered Jacks (RJs). The USOC lives on in Part 68 of the FCC rules, which has adopted parts of the USOCs and the RJ system.

Common designations are RJ-11, which is a modular telephone jack found on all modems, and RJ-45, which is the jack used by Category 5 UTP Ethernet wiring. Each jack has a number to designate its type, as well as an optional letter. Some of the letters are listed in Table E-3.

Table E-3. Meaning of letters in the USOC

Letter	Meaning
C	Surface- or flush-mounted jack
W	Wall-mounted jack
S	Single-line jack
M	Multiline jack
X	Complex jack

T1s are terminated by RJ-48X jacks, which are complex jacks because they incorporate shorting bars for automatic loopback. The RJ-48 pinout is given in Table E-4.

Table E-4. RJ-48X pinout

Pin number	Usage
1	RX ring
2	RX tip
3	Grounding
4	TX ring
5	TX tip
6	Grounding
7	Unused
8	Unused

Pins 3 and 6 are used for grounding by connecting the leads to the metal shields on the cable. Regulatory approval depends on the presence of the shield grounding.

Warning

Regulatory approval requires that the grounding pins be used. Without a properly grounded cable, your installation cannot achieve regulatory approval. Put another way, do not make these cables at home!

Pins are numbered ...